A vast history exists on the topic of transducers. A brief description follows of existing devices for acquiring pressure gradient and acoustic particle velocity measurements, which may be categorized as follows; i) motion sensors, ii) multiple or arrayed pressure sensors, or iii) a broad category of many approaches including fiber optics, liquid crystals, and others. A more comprehensive discussion of these topics may be found in “Acoustic Particle Velocity Sensors: Design, Performance, and Applications,” AIP Conference Proceedings 368, Sep. 1995 (Woodbury, New York: American Institute of Physics, (1996).
i) Motion Sensors: Geophones and accelerometers are very common types of motion sensors that respond directly to the motion of their housing. Provided the housing is rigidly connected to a medium so that the housing moves as the medium moves, the sensor may provide measurement of an acoustic or elastic wave propagating through the medium since such waves cause motion in the medium. For example, a motion sensor attached to the surface of a solid medium or buried in a viscous medium can measure acoustic or elastic waves propagating through those media. Hence, in marine seismic exploration, geophones are placed or buried on the sea bottom. However, the use of motion sensors to measure acoustic particle velocity in a non-viscous fluid medium such as water is problematic. There are two primary reasons for this. First, it is difficult to obtain a rigid connection between the housing of a motion sensor and water, or other nonviscous fluid medium, so that the sensor housing will move as the medium moves. Second, it is often the nature of water and other nonviscous fluid media to host a variety of flows and motions not related to acoustic waves propagating in the media. A motion sensor cannot distinguish motion due to propagating acoustic waves from motion due to incompressible flow or other fluid motions, but will sense all motions. Hence use of a motion sensor to measure acoustic particle velocity becomes difficult.
ii) Pressure Sensors: Hydrophones and other piezoelectric technologies are commonly employed to measure pressure variations in fluid media. Two (or more) pressure sensors placed some distance apart can be used to estimate pressure gradient. Such arrangements of hydrophones are often referred to as “differential”, “dipole”, or “pressure gradient” hydrophones. The simplest implementation uses the difference in measurements between two closely spaced hydrophones and precise knowledge of their separation. However, such methods are substantially limited in bandwidth and dynamic range compared to the individual hydrophone elements. Low frequencies, such as might be useful in marine seismic exploration, require large separation which has been achieved by towing separate hydrophone streamers at different depths. Under such circumstances, separation and relative orientation of two hydrophones may be variable, imposing an undesirable measurement system dynamic. Additionally, as with all difference measurements, the individual hydrophone and measurement systems must have excellent response uniformity to achieve adequate common mode rejection.
iii) Others: Many other methods have been proposed for pressure gradient and acoustic particle velocity measurements. Each achieves measurement objectives with varying degrees of success. However, it is widely recognized that there are many applications that would benefit from substantial improvements in this technology area.